1. Field of the Invention
The present invention relates to tone control in an image forming apparatus such as a copying machine or printer.
2. Description of the Related Art
Japanese Patent Laid-Open No. 2011-257598 discloses an image forming apparatus which forms a plurality of measurement images (patch images) having different tones, detects the densities of the patch images, and determines image processing coefficients for conversion from an input density into an output density. The image processing coefficient is tone correction data used for tone correction in image formation. Japanese Patent Laid-Open No. 2011-257598 proposes, when updating image processing coefficients, to form patch images using image processing coefficients before updating.
For example, assume that an image processing coefficient for converting an output density into a value smaller than 100% when the input density is 100% is created. In this case, if patch images are formed using image processing coefficients before updating, a patch image having a maximum density is formed in accordance with an image signal indicating a density of lower than 100%. Also, assume that the maximum density of a patch image formed at this time is lower than a target maximum density. In this case, tone correction data, that is, image processing coefficients in a range of the maximum density of a formed patch image to the target maximum density are determined by extrapolation of data representing the relationship between the density of a formed patch image and the density of an image signal used. In this case, image processing coefficients in a high density region of the maximum density of a formed patch image to the target maximum density may lose accuracy.